Optical waveguide formed by diffusing metal into substrate

ABSTRACT

The specification describes an optical waveguide device formed by diffusing a metal into a substrate which may be either a semiconductor material or a dielectric material. The substrate is first coated with a liquid composition comprising organo-metallic solutions of the desired metal and silica. The coated substrate is then heated at an elevated temperature for a period of time sufficient to cause the organic portion of the solution to decompose, thereby leaving a composite film comprising an oxide of the desired metal and SiO 2 . Upon further heating, the metal from the metal oxide diffuses into the substrate. The residual composite film may be left in place or removed.

This application is a division of application Ser. No. 911,543, filedJune 1, 1978 now U.S. Pat. No. 4,206,251.

FIELD OF THE INVENTION

The invention relates generally to a process for diffusing a metal intoa substrate consisting of either a semiconductor material or adielectric material and an optical waveguide device formed thereby. Moreparticularly, the invention relates to a process for diffusing titaniuminto LiNbO₃ crystals to form a single mode optical waveguide in whichLi₂ O out-diffusion has been prevented.

BACKGROUND OF THE INVENTION AND RELATED APPLICATION

In optical communication systems, messages are transmitted by carrierwaves of optical frequencies that are generated by sources such aslasers or light-emitting diodes. There is much current interest in suchoptical communication systems because they offer several advantages overconventional communication systems, such as a greatly increased numberof channels of communication and the ability to use other materialsbesides expensive copper cables for transmitting messages. One suchmeans for conducting or guiding waves of optical frequencies from onepoint to another is called an "optical waveguide". The operation of anoptical waveguide is based on the fact that when a medium which istransparent to light is surrounded or otherwise bounded by anothermedium having a lower refractive index, light introduced along the innermedium's axis is totally reflected at the boundary with the surroundingmedium, thus producing a guiding effect.

Certain electro-optical materials are very attractive for thisapplication since they make it possible to achieve electrical controland high-speed operation of electro-optical devices and circuits. Theuse of lithium niobate (LiNbO₃) crystals for such purposes is well-knownin the art, and is disclosed, for example, in an article entitled"Integrated Optics and New Wave Phenomenon in Optical Waveguides," P. K.Tien, in Reviews of Modern Physics, Vol. 49, No. 2 (1977), pages361-420. Lithium niobate has large electro-optic and acousto-opticcoefficients and provides low loss propagation of light within waveguidelayers or other regions within this material. Many different types ofactive channel waveguide devices using these materials have been used ina variety of electro-optical modulators and switches which arecompatible with single-mode optical fibers.

Various methods of forming high refractive index waveguides in LiNbO₃have been used in the art. They include: epitaxial growth by sputtering,epitaxial growth by melting, lithium oxide (Li₂ O) out-diffusion, andtransition metal in-diffusion. Epitaxial growth by sputtering oftenleads to films with high losses and poor electro-optical properties. Inepitaxial growth by melting, the film thickness cannot be easilycontrolled. The Li₂ O out-diffusion process generates a film which cansupport only TE polarization waves (polarization parallel to the surfaceof the waveguide structure) propagating along the X axis on a Y-cutwafer.

The in-diffusion of a transition metal, such as titanium, nickel, orvanadium, into LiNbO₃ offers a promising technique to produce planar aswell as channel waveguide structures. The in-diffusion process involvesevaporating a layer of metal, such as titanium, onto the surface of thecrystal substrate by electron-beam sputtering techniques such as thosedescribed by K. L. Chopra, "Thin Film Phenomena," Chapter 2, McGraw-HillBook Company, New York, 1969. The metal is then diffused at an elevatedtemperature, such as 900° C., for an extended period of time (e.g., 6hours). Typically, by this prior art method, a sample is cleaned andplaced in a vacuum chamber which is evacuated to a pressure of 10⁻⁸torr. Then, using an expensive and complex electron beam evaporationapparatus, titanium is evaporated onto the surface of the sample. Theseevacuation and evaporation procedures are time-consuming, requiringtypically 4 hours to complete. Thus, the prior art process has thedisadvantages of requiring expensive and complex apparatus, requiringthe maintenance of a vacuum, and being time consuming.

In addition to the problems of implementing the above described priorart process, another serious problem arises because at the hightemperature required for metal in-diffusion, loosely bound Li₂ Odiffuses out from the surface of the crystal structure. As a result ofthis Li₂ O out-diffusion, a Li₂ O-deficient planar waveguide layer isformed in the LiNbO₃ crystal in addition to the waveguides formed bymetal in-diffusion. The waveguide formed by out-diffusion can confine TEpolarization waves propagating along the X-axis on a Y-cut wafer (or theY-axis on an X-cut wafer) in an undesirable manner. (A Y-cut wafer is awafer cut perpendicular to the Y-axis of the crystal. For a moredetailed description of crystal cutting, refer to "Standards onPiezoelectric Crystals, 1949," Proceedings of the Institute of RadioEngineers, pp. 1378-1395, December 1949). In a channel wave-guidedevice, a planar out-diffusion waveguide introduces excessive cross-talkbetween guided modes from two adjacent waveguides. Cross-talk presentsparticular difficulties when trying to achieve compatibility between afiber optic communications link and optical channel waveguide switchesbased on controlled coherent coupling. The planar index increase causedby the out-diffusion of Li₂ O limits the implementation of the coherentcoupling switches to TM waveguide modes only (i.e., polarizationperpendicular to the surface of the waveguide structure). In addition,in an end-butt coupling configuration between a single mode opticalfiber and a channel waveguide, a large portion of the optical energygoes to the unwanted out-diffusion modes, which are readily excited bythe optical fiber input, and thus the coupling to the channel waveguideis effectively diminished.

The cause of the out-diffusion of Li₂ O from LiNbO₃ crystals is inherentin the particular structure of these crystals. It is well known thatLiNbO₃ crystals can be grown in a slightly non-stoichiometric form, (Li₂O)_(v) (Nb₂ O₅)_(1-v) where v ranges from 0.48 to 0.50. At the hightemperature (850° C. to 1200° C.) required for the in-diffusion oftransition metal ions in order to form a waveguide in LiNbO₃ crystals,the loosely bound Li₂ O diffuses out from the surface of the crystal. Itis known experimentally that for a small change of v in LiNbO₃, theordinary refractive index remains unchanged while the extraordinaryrefractive index increases approximately linearly as v decreases. Thereduction in the Li₂ O concentration at the surface of the crystal dueto out-diffusion thus forms a high-index layer which traps optical beamsin the direction perpendicular to the surface of the waveguidestructure.

One method for suppressing the out-diffusion of Li₂ O from LiNbO₃ andLiTaO₃ waveguide structures is disclosed in U.S. Pat. 4,196,963,assigned to the present assignee and includes exposing the LiTaO₃ andLiNbO₃ crystal structures to a Li₂ O-rich environment at sufficientvapor pressure that Li₂ O diffuses into the structure as a compensationprocess and a solid-solid surface interaction occurs. The Li₂ O-richenvironment is obtained by annealing the structure in a high puritypowder of LiNbO₃ or LiTaO₃ or in molten LiNO₃.

The invention described in this copending application is highlyeffective in suppressing Li₂ O out-diffusion from LiNbO₃ and LiTaO₃crystals. However, the present invention provides still other novel andalternative means for preventing Li₂ O out-diffusion in selectedwaveguide materials. In addition, the present invention provides animproved method for diffusing a metal into a substrate to form awaveguide structure, which overcomes many of the disadvantages ofcertain prior art processes.

SUMMARY OF THE INVENTION

The general purpose of this invention is to provide a new and improvedmethod for diffusing a metal into a substrate, and more particularly amethod for forming a Ti-diffused LiNbO₃ optical waveguide. Morespecifically, the present invention provides a new and improved meansand method for diffusing Ti into a LiNbO₃ crystal substrate to form awaveguide structure, while simultaneously preventing the out-diffusionof Li₂ O from the crystal substrate.

In order to accomplish this purpose, I have discovered and developed,among other things, a novel process in which a substrate is first coatedwith a liquid composition comprising organo-metallic solutions of thedesired metal. (The term "organo-metallic" is used herein to refer to asubstance whose molecules contain a carbon-metal linkage). The coatedsubstrate is then heated at an elevated temperature for a period of timesufficient to cause the organic portion of the solution to decompose,thereby leaving a composite film comprising an oxide of the desiredmetal and SiO₂. Upon further heating, the metal from the metal oxidediffuses into the substrate.

More specifically, I have discovered and developed a novel process inwhich a sample of LiNbO₃ crystals is coated with an organo-metallictitanium-silica film and heated at an elevated temperature for a periodof time sufficient to cause the organic material to decompose into CO₂and H₂ O, leaving a composite film comprising TiO₂ and SiO₂, and tocause Ti diffusion into the substrate to occur. The Ti from the TiO₂ inthe composite film diffuses into the LiNbO₃ crystal to form aTi-diffused LiNbO₃ optical waveguide. Simultaneously, the composite filmprevents the out-diffusion of Li₂ O from the crystal, which usuallyoccurs at elevated temperatures.

The present invention overcomes many of the disadvantages of theexpensive, complex, and time-consuming prior art process previouslydiscussed. The present invention is relatively simple, fast, andinexpensive in its implementation. The titanium-silica film may beeasily applied by known dip-coating techniques which are practiced inthe integrated electronics processing industry or by a spin coatingtechnique such as that described by W. S. DeForest, "Photoresist:Materials and Processes," Chapter 7, McGraw-Hill, New York, 1975. Inaddition, the present invention provides a high purity source of Ti, andfurther, the proportions of TiO₂ and SiO₂ therein may be varied asdesired in order to obtain the appropriate Ti concentration.

As an additional advantage, the present invention provides a film on thesurface of the crystal which prevents the diffusion of Li₂ O out of thecrystal. Furthermore, in practicing the present invention, the Li₂ Oout-diffusion is automatically eliminated without requiring a separateprocess specifically directed to that end. As previously disclosed, thisLi₂ O out-diffusion is the cause of many difficulties in the formationof optical waveguides and its elimination is highly desirable. Inaccordance with the present invention, the prevention of the Li₂ Oout-diffusion from LiNbO₃ waveguide structures prevents the formation ofunwanted waveguide modes and the associated problem of excessivecross-talk between guided modes. Further, the present invention enablesefficient end-butt coupling between a single mode optical fiber and achannel waveguide to be achieved. In addition, by optimizing both the TEand TM polarizations in the optical waveguides thus produced, thepresent invention eliminates polarization control requirements for thefabrication of fiber optic waveguides and thus eliminates or minimizesthe need for polarizers in the optical transmission line.

A noteworthy feature of the present invention is that after the opticalwaveguide has been formed, the composite residual film may be left inplace to provide a required isolation or passivation layer for thewaveguide and thus eliminating any additional processing steps requiredto form such a layer.

Accordingly, it is an object of the present invention to provide a newand improved optical waveguide device formed by diffusing a metal into aselected substrate.

A further object is to provide a new and improved optical waveguidedevice formed by diffusing Ti into a LiNbO₃ crystal substrate.

Another object is to provide a new and improved optical waveguide deviceformed by in-diffusing Ti into a LiNbO₃ substrate while simultaneouslypreventing Li₂ O out-diffusion from the substrate.

Still another object is to provide a new and improved Ti-diffused LiNbO₃single mode optical waveguide in which the formation of an unwantedplanar waveguide is prevented.

An additional object is to provide a high purity source of Ti fordiffusion.

Another object is to provide a simple, fast, and inexpensive process fordiffusing a metal into a selected substrate.

A feature of this invention is the provision of a novel compositesemiconductor-metal-dielectric optical waveguide structure wherein thedielectric layer, e.g. TiO₂ :SiO₂, is multi-functional in that it servesto prevent Li₂ O out-diffusion from the semiconductor and alsopassivates, protects and enhances optical waveguiding in the metalwaveguide layer.

These and other objects and features of the invention will become morereadily apparent in the following description of the preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1a-1d illustrate in schematic cross-section some of the majorsteps in the process of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1a, there is shown a wafer 2 of LiNbO₃ crystalsupon which has been deposited a film 4 of a liquid compositioncomprising organo-metallic solutions of Ti and silica. The film 4 isdeposited by known spin-coating techniques wherein, for example, aplurality of LiNbO₃ substrates are mounted on a suitable support table,provided with a few drops of the organo-metallic Ti-silica solution andthen rotated with circular rotational speed sufficient to centrifugallyspin the Ti-silica solution uniformly and radially across the uppersurfaces of the substrates. Then, the LiNbO₃ substrates are allowed todry before processing is resumed. Optionally, the whole wafer 2 may bedip-coated with the Ti-silica solution, in which case the layer 4 isdeposited on all sides of the wafer.

The structure of FIG. 1a is then heated at 600° C. for 4 hours to allowthe organo-metallic film to decompose and leave a composite TiO₂ -SiO₂solid film 6 shown in FIG. 1b. Next, the structure of FIG. 1b is heatedat 950° C. for 16 hours in a flowing oxygen atmosphere to allow the Tito diffuse into the LiNbO₃ wafer. The resulting waveguide structure,shown in FIG. 1c, consists of the LiNbO₃ wafer 2 into which Ti hasdiffused to form layer 8, with the residual TiO₂ -SiO₂ film 6 on thesurface of the wafer. Optionally, the residual TiO₂ -SiO₂ film 6 of FIG.1c may be removed with dilute hydrofluoric acid, to provide thewaveguide structure of FIG. 1d, consisting of the LiNbO₃ wafer 2 intowhich Ti has diffused to form layer 8.

In practicing the present invention, a Y-cut LiNbO₃ crystal, 1 inch by 1inch, was dip-coated at a pulling speed of 0.25 inches per minute withan organo-metallic liquid having the trade name "TITANIUMSILICAFILM B",purchased from Emulsitone Company, Whippany, N.J. The TITANIUMSILICAFILMB is a liquid composition comprising organo-metallic solutions of Ti andsilica and is a high purity source of Ti which contains the followinglow impurities: <0.5 ppm of Fe; <1 ppm of Cu; and <1 ppm of Na. Thecoated crystal was then placed in a high temperature furnace and heatedat 600° C. for 4 hours. When heated, the organo-metallic filmdecomposed, leaving a composite TiO₂ -SiO₂ solid film, consisting ofabout 72% TiO₂ and about 28% SiO₂ and having a thickness of about 1150angstroms. The coating was very uniform in thickness except for theedges, where a thinner coating was obtained. Next, the sample was heatedat 950° C. for 16 hours in a flowing oxygen atmosphere, which causedTi-diffusion into the LiNbO₃ crystal to occur. The residue film on thesurface was etched away readily with diluted (30%) HF. It was observedthat the area coated with TITANIUMSILICAFILM had expanded in volume andbeen raised by about 200 A. According to past experience with diffusionof electron beam evaporated Ti, the resulting diffusion area would raiseby 2.5 times to the original Ti film thickness. A 200 A increaseindicates that the amount of Ti metal diffused into the LiNbO₃ substrateis equivalent to an 80 A thick pure Ti coating.

The resulting waveguide had a very good surface appearance. One TE modeand one TM mode had been successfully excited in the waveguide using aHe-Ne laser and TiO₂ prism couplers, in the manner described by P. K.Tien, R. Ulrich, and R. J. Martin, "Modes of Propagating Light Waves inThin-Deposited Semiconductor Films," in Applied Physics Letters, Vol.14, p. 291 (1969). The guided optical beams propagated along the X-axisso that the TE and TM modal indices were determined by the index changein the extraordinary axes respectively.

In addition, no Li₂ O out-diffusion mode was observed. The waveguideloss was measured using a two-prism coupler. The first prism coupled TEpolarization He-Ne laser light into the waveguide and the second prismextracted the guided light to a detector. By monitoring the output as afunction of the prism separation, the waveguide loss was determined fromthe slope of a semi-logarithmic plot. The waveguide loss was calculatedto be 1.6 dB/cm.

It is not necessary that the residue film on the crystal surface beetched away. In fact, in certain applications it would be desirable toretain this film. Such is the case in the formation of an activewaveguide, such as a light modulator, in which a metallic electrode isused and must be isolated from the optical waveguide itself in order toavoid scattering and absorption losses. The residue film in the presentinvention can provide this type of isolation, as well as providing apassivation layer.

While the invention has been particularly described with respect to thepreferred embodiment thereof, it will be recognized by those skilled inthe art that certain modifications in form and detail may be madewithout departing from the spirit and scope of the invention. Inparticular, the scope of the invention is intended to include thediffusion of any metal which is capable of forming an organo-metallicsolution, including both transition metals such as Ti and ordinarymetals. Further, the present invention includes the diffusion of a metalinto various substrates, which may be semiconductor materials ordielectric materials. More specifically, where the present inventiondiscusses LiNbO₃ crystals in particular, it is expected that LiTaO₃crystals will function in the same manner when treated with the aboveinventive process.

What is claimed is:
 1. An optical waveguide structure wherein a lithiumniobate (LiNbO₃) substrate has a titanium-diffused waveguiding layertherein atop which is disposed a dielectric layer comprising a compositefilm of titanium oxide and silicon dioxide, formed by a processcomprising the deposition and subsequent thermal decomposition of anorgano-metallic titanium-silica film on a surface of said lithiumniobate substrate to thereby diffuse said titanium into said substrateand form said dielectric layer and simultaneously prevent unwantedout-diffusion of lithium oxide (Li₂ O) from said substrate, whereby theoptical waveguiding properties of said waveguide are enhanced.